The present disclosure relates to measuring while drilling techniques and, more particularly, to methods and apparatus for measuring bending moments in a tool string as an indicator of wellbore tortuosity, and for using such measured bending moments.
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A proportion of the current drilling activity involves directional drilling (e.g., drilling deviated and/or horizontal boreholes) to steer a well towards a target zone and increase hydrocarbon production from subterranean formations. Modern directional drilling systems generally employ a drill string having a bottom-hole assembly (BHA) and a drill bit situated at an end thereof that may be rotated by rotating the drill string from the surface, using a mud motor arranged downhole near the drill bit, or a combination of the mud motor and rotation of the drill string from the surface.
The BHA generally includes a number of downhole devices placed in close proximity to the drill bit and configured to measure certain downhole operating parameters associated with the drill string and drill bit. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices, and a resistivity measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as logging-while-drilling (“LWD”) and measuring-while-drilling (“MWD”) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled generally along predetermined desired paths identified in a well plan and typically extend through a plurality of different earth formations. In the course of such following of a well plan, a number of adjustments in the drilled well bore trajectory are required in order to make adjustments in inclination or azimuth, and even to maintain drilling in a generally linear path. As a result, during the drilling of a well there can be many adjustments in steering of the bit, and of maintaining direction of a bit, which result in changes in inclination and/or azimuth. While survey measurements performed during the drilling of the well can indicate the path of the wellbore, which may then be compared to a well-plan, such survey measurements tend to present a relatively generalized indication of the wellbore path, and can suggest a smoother wellbore profile that actually exists. For example, such survey measurements provide minimal information regarding spiraling of the wellbore, or of localized directional shifts (i.e., deflections or “dog-legs”), of magnitudes that can present greater strains upon a tool string than would be apparent from conventional survey measurements. Such spiraling or dog-legs, or other forms of well bore tortuosity, can be problematic to the drilling operations or subsequent operations within the well.